Nitrate dynamics in biosolids-treated soils. III. Significance of the organic nitrogen, a twin-pool exponential model for nitrogen management and comparison with the nitrate production from animal wastes

The extent and rate of NO−3-N production from organic fractions of different types of urban wastewater biosolids were studied in a soil incubation experiment. At least five N fractions with different NO−3 accumulation properties were identified in digested biosolids. A proportion of the organic N, and the NH4-N pool, was rapidly converted to NO−3. Other organic fractions were slowly mineralized, reflecting the degree of stabilization. Approximately 90% of the available N (60% of total N) was converted to NO−3 after 4000 d°C; 30% of the total N resisted mineralization. Immobilization of soil mineral N was the dominant process for dewatered undigested biosolids. A net release of mineral N equivalent to 15% of the total N content occurred after 4000 d°C with this type of biosolids. Composted biosolids were barely mineralized and less than 10% of the applied N was converted to NO−3 after 160 d of incubation in soil at 25°C, implying a minimal risk of NO−3 leaching. Nitrate accumulation from farm wastes followed similar patterns to the wastewater biosolids, although NO−3 production was potentially more variable compared with stabilized biosolids. The centralized collection and treatment of urban wastewater should contribute to a more consistent agricultural fertilizer. A N management model based on thermal-time is proposed for recycling biosolids to improve N utilization and minimize NO−3 leaching.